Using Nanoscale Defects to Enhance the Properties of Ultrathin, 2D Materials

According to a famous saying, nothing in this world is perfect, and this is not bad always. As part of a research at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), researchers found out how the properties of ultrathin, 2D material can be improved by nanoscale defects.

This image shows an illustration of the atomic structure of a 2D material called tungsten disulfide. Tungsten atoms are shown in blue and sulfur atoms are shown in yellow. The background image, taken by an electron microscope at Berkeley Lab’s Molecular Foundry, shows groupings of flakes of the material (dark gray) grown by a process called chemical vapor deposition on a titanium dioxide layer (light gray). (Image credit: Katherine Cochrane/Berkeley Lab)

They blended a toolbox of methods to exploit natural, nanoscale defects formed during the production of tiny flakes of a monolayer material called tungsten disulfide (WS2) and performed an in-depth evaluation of their electronic effects, which was not possible earlier.

Usually we say that defects are bad for a material,” stated Christoph Kastl, a postdoctoral researcher at Berkeley Lab’s Molecular Foundry and the lead author of the study, reported in the ACS Nano journal. “Here they provide functionality.”

Tungsten disulfide is a well-analyzed 2D material that, similar to other such 2D materials, possesses unique properties due to its atomic thinness. It is a semiconductor that is specifically famous for its efficiency to absorb and emit light.

Constituents of this class of 2D materials could function as high-efficiency computer transistors and as other electronic components. They also are important candidates for application in LED lighting and ultrathin, high-efficiency solar cells, and also in quantum computers.

It is also possible to integrate these 2D materials into new types of memory storage and data transfer, like spintronics and valleytronics, that would transform electronics by exploiting the materials in innovative ways to produce smaller and highly efficient devices.

The latest outcome is the first all-inclusive study at the Lab’s Advanced Light Source (ALS) making use of a method known as nanoARPES, which was employed by the researchers to analyze the 2D samples using X-rays. The X-rays pull out electrons in the sample, thus enabling the researchers to evaluate their energy and direction. This unraveled nanoscale defects and the way in which electrons interact with one another.

The nanoARPES facility is housed in an X-ray beamline, started in 2016, called Microscopic and Electronic Structure Observatory (MAESTRO). It is one among a few specialized beamlines at the ALS, producing light in different forms, from infrared to X-rays, for a wide range of concurrent experiments.

It’s a very big advance to get this electronic structure on small length scales,” stated Eli Rotenberg, a senior staff scientist at the ALS who was the driving force behind the development of MAESTRO and served as one of the study’s leaders. “That matters for real devices.”

The researchers also employed a method called X-ray photoelectron spectroscopy, or XPS, to analyze the chemical composition of a sample at extremely small scales, a kind of atomic force microscopy (AFM) to observe structural details down to the atomic scale, and a consolidated form of optical spectroscopy (Raman/photoluminescence spectroscopy) to analyze the interactions of light with the electrons at microscope scales.

The different techniques were used at the Molecular Foundry, where the material was produced, and at the ALS. The sample used in the research included roughly triangular, microscopic flakes, where the diameter of each flake was about 1–5 μm (one-millionth of 1 m). They were developed on top of titanium dioxide crystals with the help of a traditional layering process called chemical vapor deposition, and the defects were observed to be hugely concentrated around the edges of the flakes, an indication of the growth process. The focus of a majority of the experiments was on a single tungsten disulfide flake.

According to Adam Schwartzberg, a staff scientist at the Molecular Foundry who was a co-lead in the study, “It took a combination of multiple types of techniques to pin down what’s really going on.”

He added, “Now that we know what defects we have and what effect they have on the properties of the material, we can use this information to reduce or eliminate defects—or if you want the defect, it gives us a way of knowing where the defects are,” and offers a fresh understanding about how to propagate and develop the defects in the sample-production process.

Although the concentration of edge defects in the WS2 flakes was usually known prior to the most recent study, Schwartzberg stated that the impacts of these defects on materials performance had not been analyzed earlier in such a detailed and comprehensive manner.

The team found out that a 10% deficiency in sulfur atoms was related to the defective edge regions of the samples in comparison with other regions, and they recognized a slighter, 3% percent, sulfur deficiency toward the center of the flakes. The researchers also observed a variation in the electronic structure and the increased abundance of freely moving electrical charge-carriers related to the high-defect edge areas.

For this research, the defects were the result of a sample-growth process. The focus of future nanoARPES studies will be on samples with defects that are produced by means of chemical processing or other treatments. The researchers aspire to control the amount and types of atoms that are influenced, and the locations at which these defects are concentrated in the flakes.

Minor adjustments such as these could be vital for processes such as catalysis, which is applied to improve and speed up various significant industrial chemical production processes, as well as to investigate quantum processes that are dependent on the synthesis of individual particles that act as information carriers in electronics.

Since the analysis of WS2 and associated 2D materials is only in the genesis stage, there are various unknown factors about the roles played by particular types of defects in these materials, and Rotenberg stated that there are a plethora of possibilities for what is called as “defect engineering” in these materials.

Moreover, MAESTRO’s nanoARPES has the potential to analyze the electronic structures of stacks of different types of 2D material layers. This can allow scientists to perceive how their characteristics rely on their physical arrangement, as well as to investigate working devices that include 2D materials.

The unprecedented small scale of the measurements—currently approaching 50 nanometers—makes nanoARPES a great discovery tool that will be particularly useful to understand new materials as they are invented.

Eli Rotenberg, Senior Staff Scientist, Advanced Light Source, Lawrence Berkeley National Laboratory

MAESTRO is one of the priority beamlines to be modernized as part of the Lab’s ALS Upgrade (ALS-U) project, a major venture that will generate even brighter, extremely focused light beams for experiments.

The ALS-U project will further improve the performance of the nanoARPES technique. making its measurements 10 to 30 times more efficient and significantly improving our ability to reach even shorter length scales.

Eli Rotenberg, Senior Staff Scientist, Advanced Light Source, Lawrence Berkeley National Laboratory

NanoARPES could have a vital role to play in developing new solar technologies since it enables researchers to observe how nanoscale changes in the chemical composition, number of defects, and other structural features have an impact on the electrons that eventually govern their performance. These same matters are crucial for various other complex materials like magnets, superconductors, and thermoelectrics—which convert temperature to current and vice versa—therefore, nanoARPES will also be highly beneficial for these.

The ALS and the Molecular Foundry are both DOE Office of Science User Facilities.

Researchers from the Berkeley Lab Chemical Sciences Division, Aarhus University in Denmark, and Montana State University also took part in this study. The study was supported by the U.S. Department of Energy’s Office of Basic Energy Sciences, the DOE Early Career Grant program, Berkeley Lab’s Laboratory Directed Research and Development program, the Villum Foundation, and the German Academic Exchange Service.

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